As has been widely reported, the auxin indole-3-acetic acid (IAA) is the plant hormone responsible for the changes in root architecture induced by the plant growth-promoting rhizobacteria (PGPR) of the Azospirillum genus. Such changes increase the area of root exploration, with greater uptake of minerals and water, resulting in increased leaf area development and grain productivity. Therefore, biofertilizers containing only Azospirillum or combined with other PGPR genera are used in vegetable seeds to reduce and optimize nitrogen fertilizer dosages and to prevent its excessive use. For this reason, some molecular studies on the effects of Azospirillum during its interaction with plants have focused on the auxin response pathway. This pathway is a complex process that depends on the homeostasis, transport, perception, and signaling of auxin. In addition, it has been reported that the Target of Rapamycin (TOR) signaling pathway, activated by auxins, also participates in changes of root architecture in the model plant Arabidopsis thaliana. Due to the importance of this PGPR in various areas, from agriculture to the bioenhancement of contaminated soilsSoil, in this chapter, we will review different aspects, mechanisms, and processes involved in the remarkable effects of Azospirillum during its interaction with plants.